1. Field of the Invention
The present invention relates to a microactuator, and more particularly, to a microactuator using shape memory alloy.
2. Description of the Related Art
In general, an ink-jet printhead is a device which prints an image having a predetermined color by ejecting minor ink droplets at a desired position of a sheet of paper. Widely available printheads generally utilize a drop on demand (DOD) system for ejecting minor ink droplets onto the sheet of paper only in case of need.
Ink ejection methods for an ink-jet printhead using the DOD system include a heat-type ejection method of ejecting ink by generating bubbles in ink using a heat source, a vibration-type ejection method of ejecting ink due to the variation in volume of ink caused by the deformation of a piezoelectric body using the piezoelectric body, and an ejection method using a shape memory alloy of ejecting ink due to the variation in the volume of ink caused by the return to its original shape stored using the shape memory alloy.
In the heat-type ejection method, as a considerably large electric energy is supplied to a heater that supplies heat to a chamber of a printhead within a very short time period, heat generated by the specific resistance of the heater is used. Heat generated from the heater is transferred to ink, and the temperature of the water-soluble ink increases rapidly and exceeds a temperature that is a critical point. In this case, bubbles are generated in the ink, and due to the bubbles, pressure is applied to ambient ink, and simultaneously, ink is pushed by the volume of the bubbles. Ink to which a kinetic energy is applied due to the pressure and the variation in volume is ejected to the outside through a nozzle. The ejected ink forms ink droplets and is ejected to the target to minimize the surface energy of the ink.
In the heat-type ejection method, due to the consecutive shock caused by the pressure occurring when bubbles generated by a thermal energy break, there is a problem with durability, and it is difficult to adjust the size of ink droplets.
In the vibration-type ejection method, a voltage is applied to a diaphragm by attaching a piezoelectric material to the diaphragm so that a pressure is applied to a chamber of a printhead. The pressure is applied to the chamber of the printhead using a piezoelectric characteristic, thus ejecting ink.
Since an ink-jet printhead using the vibration-type ejection method uses a high-priced piezoelectric device, it is costly. The piezoelectric device is required to harmonize with an electrode, an insulating layer, and a protective layer. Thus, a manufacturing process thereof is difficult, and a yield thereof is low.
FIGS. 1A and 1B are cross-sectional views illustrating the operation of a conventional microactuator for an ink-jet printhead using a shape memory alloy disclosed in U.S. Pat. No. 6,123,414.
Referring to FIGS. 1A and 1B, a space portion 11 is provided to the front and rear sides of a substrate 10 while penetrating therethrough in the up and down direction, and a vibration plate 12 in which a silicon thin film 12b and a shape memory alloy 12a are sequentially stacked to cover the space portion 11 is installed on an upper surface of the substrate 10. An electrode 21a for applying current to both sides of the vibration plate 12 is installed to contact the vibration plate 12. A nozzle plate 18, in which a nozzle 19 through which ink droplets 20 are ejected is formed, is installed on the substrate 10, and a passage plate 13 in which a chamber 14 in which ink is stored is disposed between the substrate 10 and the nozzle plate 18. A passage 16 for providing a path through which ink flows into the chamber 14 is provided to the passage plate 13.
In a microactuator for an ink-jet printer having the above structure, the vibration plate 12 bends to the space portion 11 due to a residual stress of the silicon thin film 12b. Thus, the shape memory alloy 12a stacked on the vibration plate 12 also bends to the space portion 11, together with the silicon thin film 12b. If current is applied to the shape memory alloy 12a through the electrode 21a, the shape memory alloy 12a generates heat by its own resistance, raising the temperature and transforming the phase from a martensite phase to an austenite phase to be flattened.
In this case, if the temperature of the shape memory alloy 12a increases, the mechanical elasticity coefficient of the shape memory alloy 12a is increased, and the amount of elongation is increased. If the temperature of the shape memory alloy 12a decreases, the mechanical elasticity coefficient of the shape memory alloy 12a is decreased, and the amount of elongation is decreased. By repeating the above operation, the volume of the chamber 14 is varied by a displacement amount of the vibration plate 12, and the ink droplets 20 are ejected to a sheet of paper through the nozzle 19 by their kinetic energy.
In the microactuator for an ink-jet printer having the above structure, the vibration plate is comprised of a double layer, such as a silicon thin film and a shape memory alloy. Thus, it is difficult to grasp the distribution of a residual stress existing in the silicon thin film exactly, since it is difficult to grasp whether the vibration plate 12 bends to the space portion or the chamber 14 during a cooling operation according to the width and thickness of the vibration plate 12 contacting the space portion 11.
In the microactuator for an ink-jet printer having the above structure, the vibration plate of the microactuator should bend to the space portion or the chamber when required, or the width of the vibration plate should be small. It is difficult to grasp the distribution of a residual stress existing in the silicon thin film and the operating characteristic of the shape memory alloy, such that the vibration plate cannot be transformed in a desired direction. Thus, a desired function of the microactuator is not obtained, and the structural design and operating control of the microactuator is not performed precisely.